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Materials Science and Engineering A 438440 (2006) 296299
Continuous cooling transformation diagrams and propertiesof
micro-alloyed TRIP steels
M. Zhang a,b,, L. Li a, R.Y. Fu a, D. Krizan c, B.C. De Cooman
ca Department of Materials Science & Engineering, Shanghai
University, 149 Yanchang Road, 200072 Shanghai, China
b Shanghai Huizhong Automotive Manufacturing Co. Ltd. 1493 S.
Pudong Road, 200122 Shanghai, Chinac Laboratory for Iron and
Steelmaking, Ghent University, Technologiepark 903, B-9052,
Belgium
Received 8 April 2005; received in revised form 23 November
2005; accepted 12 January 2006
bstract
Continuous cooling transformation (CCT) diagrams and properties
of four kinds of low-silicon CMnSiAl transformation-induced
plasticityTRIP) steels with different carbon contents, with or
without microalloy element Ti/V, as well as a reference TRIP steel
containing 1.19 wt.%i were studied. The CCT diagrams exhibited that
as the carbon equivalent (CE) increased, it caused a shift of the
ferrite forming and pearliteorming temperatures to the right side
and the bainite forming and martensite forming to the lower
temperatures of the diagram. The microstructuralvolution obtained
from the dilatometry samples revealed that the highest cooling
rates produced fully martensitic microstructure in all cases
except
he reference TRIP steel. As the cooling rate decreased, more
ferrite and bainite were formed. The increase of CE caused the
increase of the amountf martesite in the microstructure. Tensile
test and Erichsen test of the investigated steels showed an
excellent mechanical strength and ductilityombination, with tensile
strength between 800 and 1000 MPa, total elongation of around 20%,
and a quite good formability with a dome heightf about 10 mm in all
cases.
2006 Elsevier B.V. All rights reserved.
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eywords: Micro-alloyed; TRIP steels; CCT diagram;
Microstructure; Mechan
. Introduction
In general, the increase of strength will result in the
decreasef formability or ductility. However, it has been shown that
thetrain-induced transformation phenomenon in retained austenite1]
results in an excellent combination of strength and duc-ility.
Transformation-induced plasticity (TRIP) steels have a
ultiphase microstructure consisting of ferrite, bainite,
andetained austenite. Their excellent strength-ductility
combina-ion is resulted from the presence of retained austenite,
whichransforms to martensite during plastic deformation, and
makeshe TRIP steels very attractive for structural parts in the
automo-ive industry. The chemical composition of the traditional
TRIPteel is 0.10.4 C1.5 Mn1.5 Si. In recent years, content ofi is
lowered to reduce the industrial casting and the surface-
uality or galvanising problems. Alternative low Si TRIP
steelssing Al have been developed, principally to encounter withigh
Si contents. The demand for higher strength TRIP grades
Corresponding author. Tel.: +86 21 58201188x2512; fax: +86 21
58306293.E-mail address: [email protected] (M. Zhang).
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921-5093/$ see front matter 2006 Elsevier B.V. All rights
reserved.oi:10.1016/j.msea.2006.01.128roperty; Formability
8001200 MPa) for automobile applications can be met byncreasing
the carbon content to around 0.4%, however theseigher carbon levels
introduce serious weldability problems anday cause hot rolling
difficulties, particularly for wide formats.hus the micro-alloying
concept (Ti, Nb and V) has been incor-orating in the TRIP steel in
order to keep the C content low (usu-lly
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M. Zhang et al. / Materials Science and Eng
Table 1Chemical composition of the studied steels in wt.%
Code C Mn Al Si P Ti V CE
P 0.11 1.67 0.038 1.19 0.013 0.44O 0.21 1.64 1.37 0.31 0.081
0.073 0.55Z 0.24 1.51 1.12 0.27 0.082 0.54SJ
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0.26 1.64 1.35 0.39 0.049 0.099 0.053 0.610.34 1.75 1.32 0.46
0.055 0.12 0.033 0.71
Steel P is the reference 1.19 Si-contained TRIP steel withoutny
addition of micro-alloy. Steel Z is a CMnSiAlP TRIPteel. Steel O is
a V-added micro-alloyed CMnSiAlP TRIPteel. Steel S and J are
V/Ti-added TRIP steels.
The material used in dilatometry was vacuum remelted andot
rolled. The specimen for dilatometry was of a diameter of.5 mm and
a length of 5 mm. During testing, the specimensere protected from
oxidation with a vacuum of 3 103 mbar.The heating and cooling
process in dilatometry for CCT dia-
ram constructing was as follow. The sample was heated atrate of
2 K/s to the austenitizing temperature 1200 C, heldmin; and then
cooled at a rate of 5 K/s to the soaking tem-erature 950 C, held 2
min; at last cooled to room temperaturet different cooling rates,
0.3, 1, 2, 5, 10, 30 K/s, respectively.he microstructures on the
dilatometry samples were examinedsing light optical microscopy
(LOM), and Vickers hardnessas also measured.After cold rolled to a
thickness of 1 mm, the specimens
or tensile test and Erichsen test were intercritically annealedt
800 C and austempered at 460 C to obtain TRIP-assisted
icrostructure.Tensile test was carried out using 25 mm gauge
length and
mm width specimens. Erichsen test was also carried out using0 mm
90 mm samples. The microstructures of the studied
F
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Fig. 1. CCT diagrams anineering A 438440 (2006) 296299 297
RIP steels were examined using LOM and scanning
electronicroscopy (SEM).
. Results
Fig. 1(a)(e) show the CCT diagrams constructed from
theilatometry curves, and Fig. 1(f) is the measured microhard-ess
profile of the dilato samples. From the CCT diagrams ofll the
studied steels, it can be found that as the carbon equiva-ent
increases, it causes a shift of the ferrite forming and
pearliteorming temperatures to the right side and the bainite
form-ng and martensite forming to the lower temperatures. Fromig.
1(f), the relationship between the hardness HV1 and theooling rate
can be seen. The hardness of dilatometer sam-les after different
cooling reveals that the hardness increasess the cooling rate
increases. The hardness also increases withhe increase of the
carbon equivalent. The highest hardness isained by steel J at
cooling rate of 30 K/s.
Fig. 2 exhibits the microstructures of the dilatometry sam-les.
The label at the upright corner of each image indicates theteel
code and the cooling rate. The microstructural evolutionbtained
from the dilatometry samples reveals that the highestooling rate
couples fully martensitic microstructure (Fig. 2(a),d) and (g))
except the reference steel (steel P, Fig. 2(m)). As theooling rate
decreases, more ferrite and bainite is formed. Thencrease of CE
causes the increase of the amount of martensiten the
microstructure. This can be more clearly shown in steelwith CE of
0.71 where almost fully martensitic microstruc-
ure is achieved even at a slow cooling rate of 5 K/s as shown
in
ig. 2(c).
Fig. 3 shows the mechanical properties. Obviously, the ulti-ate
tensile strength (UTS) increases with the increase of carbon
ontent, and the highest UTS of over 1 GPa is obtained in
steel
d hardness profile.
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298 M. Zhang et al. / Materials Science and Engineering A 438440
(2006) 296299
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Fig. 2. Microstructures
. On the other hand, the total elongation (EL) decreases withhe
increase of carbon content (except steel J). The investigatedteels
have an excellent combination of mechanical propertynd ductility,
with the UTS between 800 MPa and 1 GPa, EL ofround 20%. The V-added
steel shows a higher yield strengthYS) and a lower uniform
elongation (EU).Erichsen test results are shown in Fig. 4 where the
formabil-ty of the studied steels decreases slightly with the
increase ofarbon content. Steel Z, steel S, steel J show a dome
height ofbout 810 mm.
isb
e dilatometry samples.
. Discussion
As known, a variety of alloying elements is available to con-rol
the transformation behavior of steel such that, depending onhe
available processing conditions, the desired microstructures
obtained.The addition of element Al in TRIP steel can shift the
austen-te area of the equilibrium phase diagram to the right side
ashown in Fig. 5. The amount of ferrite is increased and the car-on
content of austenite is raised as well as the stability of the
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M. Zhang et al. / Materials Science and Eng
Fig. 3. Tensile test result of various steels.
Fig. 4. Dome height of three kinds of steels.
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[Aided High Strength Ferrous Alloys Proceedings, GRIPS, Ghent,
Belgium,20023, p. 199.
[4] C. Scott, in: M.A. Baker (Ed.), International Conference on
Advanced High-Strength Sheet Steels for Automotive Applications
Proceedings, WinterPark, CO, USA, 2004, pp. 181193.Fig. 5.
Equilibrium phase diagram of Steel J.ineering A 438440 (2006)
296299 299
etained austenite. As a result, the ferrite and pearlite area
arehifted to the right side, and martenite and bainite to the
loweremperature part, as shown in CCT diagrams.
In addition, the microstructure of the V-added TRIP steelhows
extremely fine grain size in steel O.
The left side of the CCT diagrams can be used to deter-ine the
microstructural evolution of the most critical zones
n the laser welds, such as fusion zone, fusion boundarynd super
critical heat-affected zone. The other part of CCTiagram
representing lower cooling rates, can also be usedn higher heat
input operations such as arc welding or hotolling.
. Conclusions
The highest cooling rates in this paper cause the formationf
full martensite. The hardness increases as the cooling
ratencreases. The hardness also increases with an increase of
thearbon content.
The studied micro-alloyed TRIP steels have an excellent
com-ination of mechanical strength, with tensile strength between00
and 1000 MPa, and total elongation of 20%.
The studied TRIP steels have a fairly good formability, withdome
height of about 10 mm.
cknowledgement
The authors are thankful to the Fund of Vanadium Interna-ional
Technical Committee for the financial support.
eferences
1] V.F. Zackay, E.R. Parker, D. Fahr, R. Bush, Trans. Am. Soc.
Met. 60 (1967)252.
2] D. Krizan, in: M.A. Baker (Ed.), 45th MSS Conference
Proceedings, vol.XLI, ISS, Chicago, USA, 912 November, 2003, pp.
437448.
3] J. Ohlert, in: B.C. De Cooman (Ed.), International Conference
on TRIP-
Continuous cooling transformation diagrams and properties of
micro-alloyed TRIP steelsIntroductionExperimental
procedureResultsDiscussionConclusionsAcknowledgementReferences